This invention relates to trigger mechanisms for semi-automatic firearms. Particularly, the invention relates to trigger mechanisms for the AR15 and M16 type rifles but with modifications may be used in other firearms. Related prior art is U.S. Pat. No. 6,131,324 issued Oct. 17, 2000 to Jewell, and U.S. Pat. No. 5,501,134 issued Mar. 26, 1996 to Milazzo. Jewell discloses a dual stage trigger assembly that allows user adjustability of sear engagement and disconnector spring force. A disconnector in Jewell is double ended with two distinct ends across the disconnector pivot point. At each end of the disconnector in Jewell is an adjustment screw. Jewell has located the first disconnector adjustment screw on the end toward the hammer. This screw will adjust the sear engagement between the trigger and hammer at the second stage let off point. On the end away from the hammer is the second adjustment screw that allows the force of the disconnector spring to be varied which will change the amount of resistance the shooter feels when pulling through the second stage to fire the weapon. Jewell's design also incorporates a unique user adjustable torsion spring that allows the user to adjust the first stage trigger pull weight. Jewell has designed a non-standard hammer spring for use with the double ended disconnector and unique torsion spring adjustable trigger. Some non-standard springs have been shown to provide reduced force over a standard hammer spring. Reduced force imparted into the hammer will allow the time of rotation of the hammer to increase over the time of rotation of an identical hammer using a stronger standard hammer spring, an undesirable situation for a shooter as the potential is increased for misalignment of firearm sights during the longer hammer fall time. The use of a standard hammer spring is also desirable from a spare parts perspective as an organization that uses M16 trigger mechanisms will not have to stock a different, special hammer spring over the standard hammer springs they now stock as spare parts.
Another dual stage user adjustable trigger is Milazzo's which allows the user to adjust sear engagement and second stage pull weight, although both adjustments are done by one screw and are not independent of each other. A distinct feature of Milazzo's trigger mechanism is the disconnector adjustment screw threadedly engaged to the trigger. Threading the screw into the trigger requires the threaded stem of the screw to bear directly on the disconnector. The cyclic sudden deceleration action of the disconnector during the weapon firing cycle has a tendency to batter the end of the threaded portion of the adjustment screw thereby changing the sear adjustment over time and distorting the screw threads such that the disconnector adjustment screw may not be easily removed for maintenance purposes.